A conventional direct current (DC) permanent magnet motor, such as is used in the automotive industry, has a single armature winding, which is energized to develop the magnetic field to provide torque to rotate the motor shaft. In a conventional single-winding DC motor, operation at two or more speeds can be achieved by including one or more resistors in the motor circuit. For example, in a two-speed motor circuit, which has a single resistor, when the resistor is bypassed, the motor receives full power (i.e. full current) from the voltage source and operates at full (or high) speed. When current is supplied to the motor through the resistor, the motor no longer receives full power, a portion of the total available power being dissipated in the resistor (as heat), and the motor therefore operates at a reduced (or low) speed. To operate a motor at a third speed (e.g. a further reduced speed) according to this arrangement, another resistor can be included in the motor circuit, and when current is supplied to the motor through the additional resistor or both resistors connected in series, the motor operates at a further reduced speed. Operating a conventional single-winding DC motor of this type at two (or three) speeds motor creates power inefficiencies and heat build-up (in the resistors) that may be undesirable in certain temperature-sensitive applications, such as where the motor is used in a fan for cooling an automotive engine.
As disclosed in U.S. Pat. No. 4,910,790 titled "TWO-SPEED MOTOR," issued on Mar. 20, 1990, DC motors having a dual-armature winding (and two commutators) are also known in the art. In a typical dual-armature DC motor, the primary winding usually has a few coil turns of light gauge wire and the secondary winding usually has several more coil turns with a heavier gauge wire, with both windings being wound onto the same lamination stack, but electrically insulated from one another (e.g. connected to separate commutators). By energizing one winding or the two windings in series, two different motor shaft speeds (high or low) can be achieved. This two-speed motor arrangement does not include the inefficiencies of the conventional two-speed single-winding DC motor (which achieves speed-control at the expense of resistive power losses), as substantially all power supplied to the motor circuit is supplied to the motor windings (and thereby used to produce torque). However, in certain applications it is desirable to have a motor that operates at three different shaft speeds (e.g. low, medium or high).
Accordingly, it would be advantageous to have an arrangement of a dual-winding DC motor with windings that can be energized independently of each other to provide for three different motor shaft speeds. It would be advantageous to have a DC motor with a switching circuit that facilitates operation at three speeds in an energy efficient manner, without undue resistive power losses and resultant heat build-up. It would also be advantageous to have a DC motor with a convenient lead arrangement and speed-matching capability suitable for a wide range of applications. It would further be advantageous to have a method of operating the DC motor that readily allows operation at three (or more) motor shaft speeds.